This invention relates to voltage regulator devices for vehicular AC generators, and more particularly to the the voltage regulator devices for AC generators with a simplified wiring system suitable to be used with internal combustion engines of agricultural and constructional machines.
FIG. 2 is a circuit diagram showing a conventional voltage regulator device for a vehicular AC generator. A voltage regulator device for a vehicular AC generator similar to that of FIG. 2 is described, for example, in Japanese Utility Model Publication (Kokoku) No. 62-30480. In FIG. 2, the AC output of the AC generator 1 including the armature coil 101 and the field coil 102 is rectified by a full-wave rectifier 2 including a main output terminal 201 and a grounded terminal 202. The output of the AC generator 1 is regulated to a predetermined voltage level by a voltage regulator circuit 3 including: voltage divider resistors 301 and 302 coupled in series, a Zener diode 303 coupled to the junction point J.sub.1 between the resistors 301 and 302, a control transistor 304, a power transistor 305 for turning on and off the field current through the field coil 102, a resistor 306 and a surge suppression diode 307. The circuit of FIG. 2 further includes a battery 4 charged by the AC generator 1 and a key switch 5, the current supply to the electric load 6 of the vehicle being controlled by a load switch 7.
The operation of the circuit of FIG. 2 is as follows. When the key switch 5 is closed before starting the engine, the base current for the power transistor 305 is supplied from the battery 4 through the key switch 5 and resistor 306, and the power transistor 305 is turned on. As a result, the field current flows from the battery 4 through the field coil 102 and the power transistor 305 to ground, and a magnetomotive force is generated by the field coil 102.
When the engine is started and the AC generator 1 is driven, an AC output voltage is induced across the armature coil 101 corresponding to the rpm of the AC generator 1, which is rectified by the full-wave rectifier 2. When the output voltage of the full-wave rectifier 2 is below a predetermined level (e.g., 14.4 V), the voltage at the junction point J.sub.1 between the resistors 301 and 302 is below the break down voltage of the Zener diode 303. The control transistor 304 is thus kept turned off. The power transistor 305 continues to be turned on and the supply of the field current to the field coil 102 is maintained. The output voltage of the AC generator 1 thus rises as the rpm of the engine increases.
When the output voltage of the AC generator 1 rises above the predetermined level (14.4 V) as a result of the increase in the rpm thereof, the voltage at the junction point J.sub.1 between the resistors 301 and 302 rises to turn on the Zener diode 303, thereby supplying base current to the control transistor 304. The control transistor 304 is thus turned on, thereby grounding the base of the power transistor 305. As a result, the power transistor 305 is turned off, so as to interrupt the field current supplied to the field coil 102. The output voltage of the AC generator 1 thus falls. When the output voltage falls below the predetermined level, the Zener diode 303 and the control transistor 304 are again turned off, and the power transistor 305 is turned on. The field coil 102 is thus energized and the output voltage of the AC generator 1 again rises.
Repeating the above operation, the output of the AC generator 1 is controlled to the predetermined level (14.4 V), and the battery 4 is thus charged to the predetermined voltage level. The surge current developed across the field coil 102 upon interruption of the field current is absorbed by the suppression diode 307.
The voltage regulator device of FIG. 2, however, has the following disadvantage. Namely, when the base current supply line A is broken or a connector thereof is disconnected from the battery 4, the supply of base current to the power transistor 305 is no longer possible. As a result, the power transistor 305 remains continuously turned off, and the AC generator 1 stops generating power. Thus the battery 4 is continually discharged without being charged by the AC generator 1 any more, until the engine is finally halted.